In some networking environments, communication networks may be formed when multiple interoperable nodes communicating over a shared medium detect the existence of other nodes. One such network operates in accordance with the well-known Multimedia over Coax Alliance (“MoCA”) MAC/PHY Specification v.1.0 or v.1.1. In such a network, nodes may function as “client” nodes. One of the client nodes is selected as a network coordinator (NC). A network typically has a single NC node and any number of client nodes. The NC node transmits beacon packets, media access plan (MAP) packets and other control information to manage the network.
MoCA networks may use in-home coaxial cable as the medium over which information is communicated. Such networks use orthogonal frequency division multiplexing (OFDM) modulation of data. OFDM is a digital multi-carrier modulation method in which a frequency band corresponding to a carrier comprises a number of closely spaced orthogonal subcarriers that are used to transport data. Data is divided into separate streams to be carried on the subcarriers. Each link between a pair of network nodes has a modulation profile in each direction that specifies the density of the modulation used on the subcarriers transmitted in that direction. For example, in accordance with one modulation profile, a first subcarrier employs 16-QAM. In accordance with 16-QAM, 16 constellation points represent one of the 16 possible values that can be represented by a four bit binary information word. A second subcarrier employs a denser modulation, such as 64-QAM (having 64 possible constellation points, each representing one of the 64 possible values of a 6 bit information word). Each of the other subcarriers has a particular modulation density which may be greater than, less than, or the same as the first and second subcarriers. In MoCA networks, binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) are considered less dense QAM modulation schemes and are also used. The denser a modulation profile, the less robust the communication. A more dense profile means more constellation points. In turn, more constellation points means more bits transmitting in the same amount of time. A signal that is transmitted using a more dense modulation scheme will be more susceptible to noise and other factors in the channel that can cause the packet error rate to be higher.
FIG. 1 illustrates one example of a communication network 200 including a plurality of network nodes 210a-g (collectively referred to as “network nodes 210”) each communicating with other nodes through a communication medium 202. Examples of the communication medium 202 include, but are not limited to, coaxial cable, fiber optic cable, a wireless transmission medium, an Ethernet cable, or the like. In one embodiment, the communication medium 202 is a coaxial cable network.
In one embodiment, network nodes 210 are communication devices within components of a home entertainment system. Such components include, for example, set top boxes (STBs), televisions (TVs), computers, DVD or Blu-ray players/recorders, gaming consoles, or the like. The nodes are coupled to each other via the communication medium 202. In some instances, the component of the home entertainment system is itself considered to be the network node.
In some embodiments, the network 200 may operate in accordance with the requirements of a MoCA network. A MoCA network dynamically assigns a network node 210 to perform the functions of a NC. Any network node 210 can function as the NC. For the sake of this example, the network node 210a performs the NC functionality. The NC forms a full mesh network architecture between each network node 210 and its peers.
Moving on to FIG. 2, each of the network nodes 210 may include a physical interface 302 including a transmitter 304 and a receiver 306. The transmitter and receiver are in signal communication with a processor 308 through a data bus 310. The transmitter 304 may include a modulator 312 for modulating data according to a quadrature amplitude modulation (QAM) scheme such as, for example, BPSK (binary phase shift keying), QPSK (quadrature phase shift keying), 8-QAM, 16-QAM, 32-QAM, 64-QAM, 128-QAM, or 256-QAM, or another modulation scheme. In addition, the transmitter may include a digital-to-analog converter (DAC) 314 for transmitting modulated signals to other network nodes 300 through the communication medium 202.
Receiver 306 may include an analog-to-digital converter (ADC) 316 for converting an analog modulated signal received from another network node 210 into a digital signal. The receiver 306 may also include an automatic gain control (AGC) circuit 318 for adjusting the gain of the receiver 306 to properly receive the incoming signal and a demodulator 320 for demodulating the received signal. One of ordinary skill in the art will understand that the network nodes 210 may include additional circuitry and functional elements not described herein.
The processor 308 may be any central processing unit (CPU), microprocessor, micro-controller, or computational device or circuit for executing instructions. As shown in FIG. 2, the processor 308 is in signal communication with a computer readable storage medium 322 through data bus 310. The computer readable storage medium may include a random access memory (RAM) and/or a more persistent memory such as a read only memory (ROM). Examples of RAM include, but are not limited to, static random-access memory (SRAM), or dynamic random-access memory (DRAM). A ROM may be implemented as a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), or the like as will be understood by those skilled in the art.
When the network 200 is initially formed (i.e., when the second new node is added to the network) or when a new network node 210 is admitted, an admission link maintenance operation (“LMO”) is performed. In accordance with the admission LMO process, the NC transmits probes to the new node. The new node receives the probes and prepares a probe report. The probe report is then sent to the NC. In addition, the new node receives probes from all of the other nodes in the network. The new node prepares probe reports for each node from which the new node received a probe. The new node also transmits probes to each of the other nodes in the network and receives probe reports from each. In general, an LMO involves transmitting probe messages formed using a predetermined bit sequence and length. The probes are transmitted from one network node to another to estimate the characteristics of the communication link between the nodes of the network. The receiving network node measures the energy of received probes and compares the measurements against predefined thresholds to determine the number of bits per subcarrier that can be supported by the communication link. The process of specifying the bit density to be used over each subcarrier is called “bit loading”. Bit loading adapts the modulation to the conditions of the network. In accordance with bit loading, modulation using a higher constellation density is used with subcarriers that have higher signal-to-noise ratio (SNR) and modulation using a lower constellation density is used with carriers that have lower SNR. The set of modulations used for all of the subcarriers on a link between a first node and a second node is referred to as a “modulation profile”. Accordingly, the modulation profile identifies the modulation used for each subcarrier on the link from one node to another node. There is a unique modulation profile associated with the link from one node to the each other nodes. The modulation profile is not symmetrical. That is, the modulation profile used on the link from a first node to a second node may be different from the modulation profile used on the link in the other direction (i.e., from the second node to the first node). Once the admission process is complete, each node will occasionally perform a “periodic LMO”. If there is interference in the communication of any of the messages that are essential to the admission process, the new node will be refused admission and will have to repeat the attempt to be admitted.
Referring to FIG. 3 which shows some of the messaging involved in a periodic LMO, in some embodiments, a probe 420 is sent from a network node 410b. The node 410b sending the probe 420 is referred to as an “LMO node”. The probe 420 may be one of a plurality of probes transmitted in support of the periodic LMO. The probe 420 is sent directly to a network node 410c. The LMO node 410b then sends a report request message 422 to the NC 410a. The NC 410a receives the request and sends the request 422 to the network node 410c. The network node 410c sends a probe report 424 back to the NC 410a. The report informs the LMO node 410b of any new bit loading to be used for subsequent packets sent to the network node 410c. Any such bit loading will take into consideration the desired error rate, such as 10−6 and the presence of interference or environmental variations which might alter the link conditions (e.g., variations in temperature or voltage since the last LMO). The sequence of events that includes probe(s), report request(s), and probe report(s) constitutes an LMO cycle. An LMO cycle is successful when all management packets (e.g., report requests, reports, media access plan packets, beacons and reservation request packets) are successfully delivered.
Interference may be present and may arise from various sources, e.g., television signals such as Advanced Television Systems Committee (ATSC) digital television broadcasting signals. An ATSC interference signal has 6 MHz bandwidth and may have various center frequencies. ATSC interference can occupy a bandwidth corresponding to some of the subcarriers. As used herein, the term “sustained interference” refers to interference that is substantially non-varying and present over a relatively long period of time (e.g., several minutes). Interference may also be dynamic interference, as opposed to sustained interference. Dynamic interference may be caused by turning on a television station transmitter, for example and may cause high packet error rate in excess of the specified error rate (e.g., 10−6). Messages (e.g., packets) that are transmitted using OFDM subcarriers may be affected by interference. Such interference can result in errors. In particular, one problem that occurs in MoCA and similar networks is that when interference is present, some messages may not get through. For example, dynamic interference may cause incorrect transmission from the network node 410c to the NC 410a, as shown by a cross (X) and dashed lines corresponding to this link in FIG. 3. Accordingly, adjustments to the bit loading might not occur if the probe report 424 is not successfully conveyed to the network node 410b. After a predetermined amount of time, the LMO node will once again send a report request 426. The report request 426 will be relayed by the NC to the network node 410c. The network node 410c in turn will unsuccessfully attempt to transmit the probe report 428 to the NC again.
Another case, shown in FIG. 4, in which interference occurs on the link from the NC 410a to the LMO network node 410b is when the LMO node 410b sends a probe request 422 successfully to the network node 410c via the NC 410a. Node 410c then relays the report 424. However, the report 424 is unsuccessfully relayed to the LMO node 410b due to interference, as shown by dashed lines and a cross in FIG. 4. The LMO node 410b, which does not know of the interference, sends another report request 426, The Node 410c then relays another report 428, and the pattern may repeat, with a responsive report 424, 428 not being conveyed correctly on the link between the NC 410a and the LMO node 410b each time.
As can be seen by the examples of FIGS. 3 and 4, interference may cause a predetermined class of messages (e.g., management packets such as the probe request 422 or the probe report 424) to be received incorrectly. Such management packets may be relatively large compared to other messages and may therefore occupy a substantial fraction (or all) of the subcarriers transmitted between nodes. As a result, such management packets may be transmitted using subcarriers that are affected by the interference. On the other hand, smaller messages, such as heartbeat messages, may be transmitted using fewer subcarriers. In particular, these smaller messages may be transmitted and received successfully without using any subcarriers affected by the interference. Because heartbeat messages may be transmitted and received successfully, the network is kept alive but is ineffective for transmitting LMO messages (thus the communication becomes deadlocked). Because management packets (such as probe reports that contain adjusted bit loading information in the face of newly turned-on interference) are not received accurately enough to be useful, a modulation profile for an affected node may not be adjusted correctly. Hence a higher packet error rate may persist. As a result of the heartbeats not detecting the failure of the communication, the network 200 may not dissolve and reform, as would otherwise be the case. Rather, the network may continue to operate poorly for larger messages. It should be understood that interference may also impede the transmission of smaller packets. Therefore, even such smaller packets may not be communicated accurately.
Accordingly, there is a need for a method and apparatus for ensuring that LMO the network can properly adjust the modulation profiles and maintain the proper operation of the network in the face of interference.